1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an active matrix liquid crystal display device in a normally white mode.
2. Description of the Related Art
Characteristics of a liquid crystal display device are that it is thin and light, it can be driven at a low voltage, and display a colored image. This type of device has recently been employed as a display device for a personal computer, a wordprocessor, and the like. In particular, an active matrix liquid crystal display device including a switching element for each of pixels does not deteriorate in contrast or response even though the pixels are increased in number. It allows a half-tone display and is the most suitable for use in color television sets and office automation equipment.
FIG. 10 shows a circuit equivalent to a conventional active matrix liquid crystal display device in which thin-film transistors (TFT) are used as switching elements. The liquid crystal display device includes two substrates (an array substrate and an opposed substrate) formed of a transparent insulation material such as glass, and a liquid crystal layer 8 interposed between these substrates. Pixels each having a TFT 3 and a transparent pixel electrode 4 are arranged in matrix on the array substrate. The transparent pixel electrode 4 is connected to the source of the TFT 3, and an address line 1 and a data line 2 are connected to the gate and drain thereof, respectively. An opposed electrode 7 is arranged on the opposed substrate so as to oppose to the transparent pixel electrodes 4. In the liquid crystal display device so constituted, an address signal and a data signal are supplied to the address line 1 and data line 2 at predetermined timing, respectively, with the result that voltages corresponding to display can be selectively applied to the pixel electrodes 4. The orientation of the liquid crystal layer 8--i.e. the light transmittance thereof--can be controlled by a difference in potential between the opposed electrode 7 and pixel electrodes 4, thereby allowing an arbitrary display.
Liquid crystal in a twisted nematic mode is generally used for the liquid crystal layer 8, and a polarizing plate is formed on each of the outer faces of the substrates. Two display modes--a normally white mode and a normally black mode--can be achieved in accordance with the arrangement of the polarizing plate. More specifically, when no voltage is applied to the liquid crystal layer 8, the maximum light transmittance represents the normally white mode, whereas the minimum light transmittance indicates the normally black mode. However, since, in the normally black mode, a slight difference in thickness between liquid crystal layers causes irregularities in the minimum light transmittance, and the thicknesses of the optimum liquid crystal layers differ from one another with the wavelength of light transmitted through the liquid crystal layers, high contrast is difficult to achieve. Therefore, the normally white mode is more frequently used.
The process for manufacturing switching elements such as the TFT is extremely complicated, and it is therefore very difficult to form defect-free pixels; some defective pixels are always included in the finished pixels. Defective pixels include a bright spot defective pixel which cause a bright spot defect on the screen of the liquid crystal display device when the screen is displayed black. The bright spot defective pixel degrades the display quality most. Basically, the the bright spot defect is caused in the liquid crystal display device in the normally white mode in a case where a voltage of a level which would cause the light transmittance of the liquid crystal layer to vary is not applied to the pixel electrode of the defect. Since there are many causes for this, the bright spot defect cannot be completely eliminated even by use of various types of redundant structures.
As described above, the conventional active matrix liquid crystal display device in the normally white mode has a drawback in which its display quality is greatly degraded because of bright spot defective pixels.